1. Field of the Invention
The present invention relates to a thin-film magnetic head comprising a magnetoresistive element, and to a head gimbal assembly and a hard disk drive each incorporating the thin-film magnetic head.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of hard disk drives has increased. A widely used type of thin-film magnetic head is a composite thin-film magnetic head that has a layered structure in which a write (recording) head having an induction-type electromagnetic transducer for writing and a read (reproducing) head having a magnetoresistive (MR) element for reading are stacked on a substrate.
MR elements include: anisotropic magnetoresistive (AMR) elements utilizing an anisotropic magnetoresistive effect; giant magnetoresistive (GMR) elements utilizing a giant magnetoresistive effect; and tunnel magnetoresistive (TMR) elements utilizing a tunnel magnetoresistive effect.
It is required that the characteristics of a read head include high sensitivity and high output capability. GMR heads incorporating spin-valve GMR elements have been mass-produced as read heads that satisfy such requirements.
A typical spin-valve GMR element incorporates: a nonmagnetic conductive layer having two surfaces facing toward opposite directions; a free layer disposed adjacent to one of the surfaces of the nonmagnetic conductive layer; a pinned layer disposed adjacent to the other of the surfaces of the nonmagnetic conductive layer; and an antiferromagnetic layer disposed adjacent to one of the surfaces of the pinned layer farther from the nonmagnetic conductive layer. The free layer is a layer in which the direction of magnetization changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer in which the direction of magnetization is fixed. The antiferromagnetic layer is a layer that fixes the direction of magnetization in the pinned layer by means of exchange coupling with the pinned layer.
As disclosed in the Published Unexamined Japanese Patent Application Heisei 8-7235 (1996), the Published Unexamined Japanese Patent Application 2000-276714, and the Published Unexamined Japanese Patent Application 2000-113418, for example, spin-valve GMR elements have been proposed, each of the GMR elements including a pinned layer in which the direction of magnetization is fixed independently of the function of any other layer such as the antiferromagnetic layer, and including no antiferromagnetic layer. In each of the GMR elements disclosed in the above-mentioned publications, the pinned layer incorporates two ferromagnetic layers and a thin coupling layer disposed between the ferromagnetic layers. In the pinned layer, the two ferromagnetic layers are coupled to each other antiferromagnetically, that is, coupled to each other such that the directions of magnetization therein are anti-parallel.
As disclosed in the Published Unexamined Japanese Patent Application 2002-100011, for example, in a GMR head, typically, the GMR element is located between two shield layers disposed on top and bottom thereof. An insulating film is provided between the GMR element and each of the shield layers. Bias field applying layers are disposed on both sides of the GMR element that are opposed to each other in the direction of track width. The bias field applying layers apply a bias magnetic field to the free layer. The bias magnetic field directs the magnetization in the free layer to the direction of track width while no signal magnetic field sent from the recording medium is applied to the free layer. The magnetization in the pinned layer is fixed to the direction orthogonal to a medium facing surface of the head that faces toward the recording medium. Consequently, an angle of 90 degrees is maintained between the direction of magnetization in the pinned layer and the direction of magnetization in the free layer while no signal field sent from the recording medium is applied to the free layer. If a signal field in the direction orthogonal to the medium facing surface is sent from the recording medium and applied to the GMR head, the direction of magnetization in the free layer is changed, and the angle between the direction of magnetization in the pinned layer and the direction of magnetization in the free layer is thereby changed. The electrical resistance of the GMR element is changed by this angle. Therefore, it is possible to read data stored on the medium by detecting the change in electrical resistance of the GMR element.
To achieve higher recording density, it is preferred that the read gap length, that is, the distance between the two shield layers of the GMR head, is small. The above-mentioned GMR element that includes no antiferromagnetic layer is suitable for achieving higher recording density since this GMR element is capable of making the read gap length smaller, compared with the GMR element including the antiferromagnetic layer.
In the GMR head using the GMR element incorporating the pinned layer in which the direction of magnetization is fixed independently of the function of any other layer, an induced magnetic anisotropy is given to the pinned layer by magnetizing the pinned layer such that the direction of magnetization in the pinned layer is orthogonal to the medium facing surface. For this GMR head it is also required to magnetize the bias field applying layers such that the magnetization in the bias field applying layers is directed to the direction of track width. The magnetic field required for magnetizing the pinned layer is greater than the magnetic field required for magnetizing the bias field applying layers. For example, the magnetic field required for magnetizing the pinned layer is about 5 to 10 kOe (5×79.6 kA/m to 10×79.6 kA/m) while the magnetic field required for magnetizing the bias field applying layers is about 1.5 kOe (1.5×79.6 kA/m). Therefore, magnetizing of the bias field applying layers should be performed after magnetizing the pinned layer.
In the GMR head using the GMR element incorporating the pinned layer in which the direction of magnetization is fixed independently of the function of any other layer, the direction of magnetization in the pinned layer should not be changed when the bias field applying layers are magnetized. In the actual manufacturing process of such heads, however, it is difficult to sufficiently increase the induced magnetic anisotropy of the pinned layer. As a result, the directions of magnetization in the pinned layers in some of the heads are changed when the bias field applying layers are magnetized. Such heads in which the directions of magnetization in the pinned layers are changed exhibit a reduction in output. If the reduction in output is beyond the permissible range, the head is rated as a nonconforming product.
As disclosed in the Published Unexamined Japanese Patent Application 2000-113418, it is known that tensile stress in the direction orthogonal to the medium facing surface exists as internal stress in the GMR element in the GMR head. This publication thus discloses that the ferromagnetic film making up the pinned layer should have a positive magnetostriction constant. If the ferromagnetic film making up the pinned layer has a positive magnetostriction constant, a magnetic anisotropy that directs the magnetization to the direction orthogonal to the medium facing surface is generated in the pinned layer, in response to the above-mentioned tensile stress, by the inverse magetostrictive effect. This magnetic anisotropy enhances the fixing of the direction of magnetization in the pinned layer.
However, in the GMR head using the GMR element incorporating the pinned layer in which the direction of magnetization is fixed independently of the function of any other layer, it is difficult to sufficiently enhance the fixing of the direction of magnetization in the pinned layer only by making the ferromagnetic film making up the pinned layer have a positive magnetostriction constant.